1. Field of the Invention
The present invention generally relates to enhanced methods of diameter selection for nanotube growth by pretreatment of the at least one catalyst particle from which the at least one nanotube grows.
2. Background
Carbon nanotubes are nanoscale high-aspect-ratio cylinders consisting of hexagonal rings of carbon atoms that may assume either a semiconducting electronic state or a conducting electronic state. Semiconducting carbon nanotubes have been used to form hybrid devices, such as hybrid FET's. In particular, FET's have been fabricated using a single semiconducting carbon nanotube as a channel region. Typically, ohmic contacts at opposite ends of the semiconducting carbon nanotube extending between a source electrode and a drain electrode situated on the surface of a substrate.
Accordingly, much attention has been given to the use nanomaterials in semiconductor devices.
Many methods exist for forming and/or creating nanotubes and nanotube arrays. A conventional method of forming carbon nanotubes utilizes a chemical vapor deposition (CVD) process. Specifically, the CVD process directs a flow of a carbonaceous reactant to a catalyst material located on the substrate, where the reactant is catalyzed to synthesize carbon nanotubes. The carbon nanotubes are capable of being lengthened by insertion of activated carbon atoms at the interface with the catalyst material. Typically, the carbon nanotubes are then collected for an end use or further processing.
In defining the size and conformation of single-wall carbon nanotubes, the system of nomenclature described by Dresselhaus et al., Science of Fullerenes and Carbon Nanotubes, 1996, San Diego: Academic Press, Ch. 19, is commonly used. Single-wall tubular fullerenes are distinguished from each other by a double index (n, m), where n and m are integers that describe how to cut a single strip of hexagonal graphite such that its edges join seamlessly when the strip is wrapped onto the surface of a cylinder. When n=m, the resultant tube is said to be of the “arm-chair” or (n, n) type, since when the tube is cut perpendicularly to the tube axis, only the sides of the hexagons are exposed and their pattern around the periphery of the tube edge resembles the arm and seat of an arm chair repeated n times. When m=0, the resultant tube is said to be of the “zig zag” or (n,0) type, since when the tube is cut perpendicular to the tube axis, the edge is a zig zag pattern. Where n≠m and m≠0, the resulting tube has chirality. The electronic properties are dependent on the conformation, for example, arm-chair tubes are metallic and have extremely high electrical conductivity. Other tube types are metallic, semi-metals or semi-conductors, depending on their conformation. Regardless of tube type, all single-wall nanotubes have extremely high thermal conductivity and tensile strength.
Single-wall carbon nanotubes have been made in a DC arc discharge apparatus by simultaneously evaporating carbon and a small percentage of Group VIIIb transition metal from the anode of the arc discharge apparatus. These techniques allow production of only a low yield of carbon nanotubes, and the population of carbon nanotubes exhibits significant variations in structure and size.
Another method of producing single-wall carbon nanotubes involves laser vaporization of a graphite substrate doped with transition metal atoms (such as nickel, cobalt, or a mixture thereof) to produce single-wall carbon nanotubes. The single-wall carbon nanotubes produced by this method tend to be formed in clusters, termed “ropes,” of about 10 to about 1000 single-wall carbon nanotubes in parallel alignment, held by van der Waals forces in a closely packed triangular lattice. Nanotubes produced by this method vary in structure, although one structure tends to predominate. Although the laser vaporization process produces an improved yield of single-wall carbon nanotubes, the product is still heterogeneous, and the nanotubes tend to be too tangled for many potential uses of these materials. In addition, the laser vaporization of carbon is a high energy process.
Another way to synthesize carbon nanotubes is by catalytic decomposition of a carbon-containing gas by nanometer-scale metal particles supported on a substrate. The carbon feedstock molecules decompose on the particle surface, and the resulting carbon atoms then precipitate as part of a nanotube from one side of the particle. This procedure typically produces imperfect multi-walled carbon nanotubes.
Another method for production of single-wall carbon nanotubes involves the disproportionation of CO to form single-wall carbon nanotubes and CO2 on alumina-supported transition metal particles comprising Mo, Fe, Ni, Co, or mixtures thereof. This method uses inexpensive feedstocks in a moderate temperature process. However, the yield is limited due to rapid surrounding of the catalyst particles by a dense tangle of single-wall carbon nanotubes, which acts as a barrier to diffusion of the feedstock gas to the catalyst particle surface, limiting further nanotube growth.
Control of ferrocene/benzene partial pressures and addition of thiophene as a catalyst promoter in an all gas phase process can produce single-wall carbon nanotubes. However, this method suffers from simultaneous production of multi-wall carbon nanotubes, amorphous carbon, and other products of hydrocarbon pyrolysis under the high temperature conditions necessary to produce high quality single-wall carbon nanotubes.
A method for producing single-wall carbon nanotubes has been reported that uses high pressure CO as the carbon feedstock and a gaseous transition metal catalyst precursor as the catalyst. (“Gas Phase Nucleation and Growth of Single-Wall Carbon Nanotubes from High Pressure Carbon Monoxide,” International Pat. Publ. WO 00/26138, published May 11, 2000, incorporated by reference herein in its entirety). This method possesses many advantages over other earlier methods. For example, the method can be done continuously, and it has the potential for scale-up to produce commercial quantities of single-wall carbon nanotubes. Another significant advantage of this method is its effectiveness in making single-wall carbon nanotubes without simultaneously making multi-wall nanotubes. Furthermore, the method produces single-wall carbon nanotubes in high purity, such that less than about 10 wt % of the carbon in the solid product is attributable to other carbon-containing species, which includes both graphitic and amorphous carbon.
As grown nanotubes, particularly carbon nanotubes, typically range from a few to tens of nm in diameter, and are as long as a few nanometers in length. Because of its one-dimensional electronic properties due to this shape anisotropy, the carbon nanotube characteristically has a maximum current density allowing the flowing of current without disconnection of 1,000,000 A per square centimeter, which is 100 times or more as high as that of a copper interconnect. Further, with respect to heat conduction, the carbon nanotube is ten times as high in conductivity as copper.
In terms of electric resistance, it has been reported that transportation without scattering due to impurities or lattice vibration (phonon) can be realized with respect to electrons flowing through the carbon nanotube. It is known that resistance per carbon nanotube, in various instances, is approximately 6.45 kΩ. However, other resistances are contemplated in various embodiments of the present invention.
Further desirable attributes of a carbon nanotube electrode material include such factors as high surface area for the accumulation of charge at the electrode/electrolyte interface, good intra- and interparticle conductivity in the porous matrices, good electrolyte accessibility to the intrapore surface area, chemical stability and high electrical conductivity. Commonly used carbonaceous material used for the construction of carbon nanotubes include such materials as activated carbon, carbon black, carbon fiber cloth, highly oriented pyrolytic graphite, graphite powder, graphite cloth, glassy carbon, carbon aerogel, and/or the like.
Typically, nanotubes can be classified into horizontal architectures and vertical architectures. Horizontal nanotubes exhibit carrier flow from source to drain in a direction parallel to the horizontal plane of the substrate on which they are formed. Vertical nanotubes exhibit carrier flow from source to drain in a direction vertical to the horizontal plane of the substrate on which they are formed.
It is commonly understood that vertical nanotubes provide and/or allow for a shorter switching time because channel length for vertical nanotubes does not depend on the smallest feature size resolvable by, for example, lithographic equipment and methods. Therefore, vertical nanotubes possess a higher power handling capacity than typical horizontal nanotubes.
Previous studies have shown that carpets (forests) of single-walled carbon nanotubes can be readily grown at atmospheric pressures with controlled mixtures containing various hydrocarbons and also in the presence of hydrogen and various hydrocarbons at sub-atmospheric pressures with activation of gas mixtures via plasma formation by microwave or RF discharges. In all cases, however, production of small diameter SWNT was not optimized with the use of substrate heating in the presence of an activated gas. Previous studies have also shown that hot filament activation of gas mixtures of hydrogen and hydrocarbons activates the growth of multi-walled carbon nanotubes in the presence of metal catalyst particles. Hata, et al., Science 2004, 306, 1362; Gyula, et al., J. Phys. Chem. B 2005, 109, 16684; Zhang, et al., PNAS 2005, 102, 16141; Iwasaki, et al., J. Phys. Chem. B 2005, 109, 19556; Zhong, et al, J. Appl. Phys, 2005, 44, 1558; Maruyama, et al., I 12005, 403, 320; Huang et al., J. Am Chem. Soc. 2003, 125, 5636.
Recent studies have shown that small diameter carbon nanotubes (1-2 nm in diameter) can be grown from metal catalyst particles deposited on surfaces. (1) This is accomplished by causing nanotube nucleation to occur at temperatures where catalyst particles are immobile. This permits the nucleation of nanotubes whose diameters are as small as the original metal catalyst particles.
However, in the prior art, a process requiring and/or desiring a nanotube of diameter less than 1 nm would use a catalyst particle, such as a metal catalyst particle, with a diameter less than 1 nm. In this manner, a design characteristic for the grown nanotube is the diameter and/or size of the catalyst particle and process and/or product can be varied by varying catalyst particle size. In fact, in the prior art, the diameter of the catalyst particles is a limiting factor on the growth of a nanotube.
Accordingly, the art field is in search of improved methods of manufacturing semiconductor devices out of nanotube material, such as carbon nanotubes, especially improved methods of growth and/or production of size controlled and/or selected arrays and/or forests of nanotubes.